Exhaust device of motorcycle

ABSTRACT

An exhaust device for a saddle-type vehicle having a pipe section with respect to a vehicle body and housing a catalyzer therein, and a muffler connected to a rear end of the pipe section includes an upstream oxygen sensor positioned upstream of the catalyzer and a downstream oxygen sensor positioned downstream of the catalyzer. The downstream oxygen sensor is disposed in either a position overlapping a main stand for supporting a motorcycle as viewed from below the vehicle body when the main stand is stored, or a position overlapping a stand receiver for abutting against the main stand as viewed from below the vehicle body when the main stand is stored. The pipe section includes a larger-diameter portion having an increased diameter for housing the catalyzer therein.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT International ApplicationNo. PCT/JP2016/057469, filed on Mar. 9, 2016, which claims priorityunder 35 U.S.C. 119(a) to Patent Application No. 2015-060880, filed inJapan on Mar. 24, 2015, all of which are hereby expressly incorporatedby reference into the present application.

TECHNICAL FIELD

The present invention relates to an exhaust device for motorcycle, andmore particularly to an exhaust device for motorcycle which is capableof detecting the degree of deterioration of a catalyst due to aging orthe like.

BACKGROUND ART

Heretofore, there has been known an exhaust device for vehicles whichincludes a catalyzer for purifying an exhaust gas that is disposedmidway in an exhaust passage extending from an exhaust pipe connected tothe exhaust port of an engine to a muffler on a rear portion of avehicle body. In order to combine such an exhaust device with an oxygensensor (O₂ sensor) for detecting the oxygen concentration of the exhaustgas and feeding it back to engine control, it is necessary to consideran optimum position for the oxygen sensor as well as its relationship tothe catalyzer.

Patent Document 1 discloses an exhaust device for saddle-type vehiclesin which exhaust pipes connected to a plurality of exhaust port of amulti-cylinder engine are collected together below the engine andcoupled to a single intermediate pipe, a muffler being connected to theintermediate pipe, a catalyzer is housed in a larger-diameter portiondisposed midway in the intermediate pipe, and an oxygen sensor isinstalled on a portion of the intermediate pipe upstream of thecatalyzer.

CITATION LIST Patent Literature

Patent Document 1: JP 2010-255514 A

SUMMARY OF INVENTION Technical Problem

In recent years, there have been demands for deterioration detection forcontinuously monitoring whether an installed catalyzer is exerting itsdesired performance, as well as an increase in the purifying performanceof the catalyzer. The deterioration detection needs two oxygen sensorsdisposed at least upstream and downstream of the catalyzer. Onsaddle-type vehicles where an extra space is small and an exhaust deviceis exposed outwardly, it is necessary to consider not only the functionof the deterioration detection but also the layout of oxygen sensors ina manner to meet conditions about how the oxygen sensors affect theappearance and should be prevented from undergoing failures.

It is an object of the present invention to provide an exhaust devicefor motorcycle which will solve the above tasks of the existingtechnology and which has two oxygen sensors, disposed in suitablepositions, for making it possible to detect deterioration of acatalyzer.

Solution to Problems

To achieve the afore-mentioned object, the present invention has a firstfeature in that an exhaust device (20) for a motorcycle having a pipesection (19) extending below an engine (E) for guiding an exhaust gastherefrom rearwardly with respect to a vehicle body and housing acatalyzer (C) therein, and a muffler (26) connected to a rear end ofsaid pipe section (19), comprising: an upstream oxygen sensor (U)positioned upstream of said catalyzer (C) and a downstream oxygen sensor(D) positioned downstream of said catalyzer (C), wherein said downstreamoxygen sensor (D) is disposed in either a position overlapping a standdevice (22, 140) for supporting a vehicle (1) as viewed from below thevehicle body when said stand device (22, 140) is stored, or a positionoverlapping a stand receiver (90) for abutting against said stand device(22, 140) as viewed from below the vehicle body when said stand device(22, 140) is stored, said pipe section (19) includes a larger-diameterportion (61) having an increased diameter for housing said catalyzer (C)therein, said larger-diameter portion (61) is positioned below acylinder block (43) of said engine (E) and in front of a crankcase (52)of said engine (E), said engine (E) has a cylinder (43) whose cylinderaxis (Os) is inclined forwardly of the vehicle body with respect to avertical direction (V), said larger-diameter portion (61) is disposedsuch that said larger-diameter portion (61) has an axis (Oc)substantially parallel to said cylinder axis (Os), and said upstreamoxygen sensor (U) and said downstream oxygen sensor (D) are mountedoutwardly from the insides of said pipe section (19) and said muffler(26) in the vehicle widthwise direction.

The present invention has a second feature in that said upstream oxygensensor (U) includes a heaterless oxygen sensor (100) free of a heaterfunction and said downstream oxygen sensor (D) includes a heater-mountedoxygen sensor (110) with a heater function.

The present invention has a third feature in that said stand device (22,140) includes a main stand, and said stand receiver (90) is provided onsaid muffler (26).

The present invention has a fourth feature in that said stand receiver(90) includes a plate-like member having upstanding wall surfaces (90 c)disposed on front and rear ends of a bottom surface (90 a) and having asubstantially U-shaped cross section.

The present invention has a fifth feature in that said upstream oxygensensor (U) is mounted on an outlet pipe (60) positioned upstream of saidlarger-diameter portion (61).

The present invention has a sixth feature in that said outlet pipe (60)includes a smaller-diameter portion (66) connected to an exhaust port ofsaid engine (E), said upstream oxygen sensor (U) being mounted on saidsmaller-diameter portion (66), and a bent portion (67) connected to saidsmaller-diameter portion (66) for bending a direction in which theexhaust gas is discharged rearwardly with respect to the vehicle bodyand connected to said larger-diameter portion (61), and said bentportion (67) has a recessed-projected portion (68) for dispersing theexhaust gas.

The present invention has an eighth feature in that said larger-diameterportion (61) includes a front portion as an outer shell (76) of anassembled hollow structure and a rear portion (83) as an integralcomponent, and said catalyzer (C) is disposed in said front portion.

The present invention has a ninth feature in comprising a catalystdiagnosing unit (72) configured to diagnose and detect a deterioratedstate of said catalyzer (C) on the basis of sensor outputs from saidupstream oxygen sensor (U) and said downstream oxygen sensor (D),wherein when said catalyst diagnosing unit (72) diagnoses said catalyzer(C) as being deteriorated, an indicator (74) is activated to indicatethe deteriorated state to an occupant.

The present invention has a tenth feature in that said stand receiver(90) is fixed in a position near a front end of a tapered portion (85)covering a joint pipe (84) for introducing the exhaust gas into saidmuffler (26).

The present invention has a eleventh feature in that said stand device(22, 140) includes a gusset (133, 134, 135, 136, 150) increasing therigidity of the stand device (22, 140) and protecting said downstreamoxygen sensor (D).

The present invention has a twelfth feature in that said main stand (22)for abutting against said stand receiver (90) has a portion including arubber damper (130) mounted on a mount (22 d) fixed to a main pipesection (22 a) of said main stand (22).

Advantageous Effects of Invention

According to the first feature, an upstream oxygen sensor (U) positionedupstream of said catalyzer (C) and a downstream oxygen sensor (D)positioned downstream of said catalyzer (C) are comprised, wherein saiddownstream oxygen sensor (D) is disposed in either a positionoverlapping a stand device (22, 140) for supporting a vehicle (1) asviewed from below the vehicle body when said stand device (22, 140) isstored, or a position overlapping a stand receiver (90) for abuttingagainst said stand device (22, 140) as viewed from below the vehiclebody when said stand device (22, 140) is stored, said pipe section (19)includes a larger-diameter portion (61) having an increased diameter forhousing said catalyzer (C) therein, said larger-diameter portion (61) ispositioned below a cylinder block (43) of said engine (E) and in frontof a crankcase (52) of said engine (E), said engine (E) has a cylinder(43) whose cylinder axis (Os) is inclined forwardly of the vehicle bodywith respect to a vertical direction (V), said larger-diameter portion(61) is disposed such that said larger-diameter portion (61) has an axis(Oc) substantially parallel to said cylinder axis (Os), and saidupstream oxygen sensor (U) and said downstream oxygen sensor (D) aremounted outwardly from the insides of said pipe section (19) and saidmuffler (26) in the vehicle widthwise direction. Therefore, even if theexhaust device is arranged below the engine for achieving a neatappearance, the downstream oxygen sensor can be protected using existingcomponents of the vehicle. Specifically, the downstream oxygen sensor isconcealed by the stand or the stand receiver in a bottom view of thevehicle body, and hence is protected from jumping pebbles and splashingwater. Furthermore, the bent portion of the outlet pipe makes itpossible to place the larger-diameter portion in a position closer tothe engine, allowing the catalyzer to exhibit its performancesufficiently. The recessed-projected portion disperses the exhaust gaswhich tends to gather in one side of the tubular passage due to the bentportion, so that the exhaust gas can pass through the catalyzer in itsentirety. Furthermore, two oxygen sensors are mounted outwardly from theinside of the pipe section in the vehicle widthwise direction, so thatthe oxygen sensor is to be hard to visible. As a result, the appearanceof the motorcycle can be improved.

According to the second feature, said upstream oxygen sensor (U)includes a heaterless oxygen sensor (100) free of a heater function andsaid downstream oxygen sensor (D) includes a heater-mounted oxygensensor (110) with a heater function. Therefore, the upstream oxygensensor is disposed in an area close to the engine as a heaterless oxygensensor which is less costly. The downstream oxygen sensor that is remotefrom the heat source may be a heater-mounted oxygen sensor that canquickly be activated. It is possible to protect the downstream oxygensensor from jumping pebbles and splashing water while achieving a neatappearance, using existing components.

According to the third feature, said stand device (22, 140) includes amain stand, and said stand receiver (90) is provided on said muffler(26). Therefore, as the receiver for the main stand is already formedfirmly, an existing component may be diverted as the receiver forprotecting the downstream oxygen sensor from below.

According to the fourth feature, said stand receiver (90) includes aplate-like member having upstanding wall surfaces (90 c) disposed onfront and rear ends of a bottom surface (90 a) and having asubstantially U-shaped cross section. Therefore, the downstream oxygensensor can be covered in its downward, forward, and rearward directionswith the bottom surface and wall surfaces of the stand receiver.

According to the fifth feature, said upstream oxygen sensor (U) ismounted on an outlet pipe (60) positioned upstream of saidlarger-diameter portion (61). Therefore, if the upstream oxygen sensoris also housed in the larger-diameter portion, then the larger-diameterportion becomes larger in size, possibly increasing the production cost.However, since only the catalyzer is housed in the larger-diameterportion, the larger-diameter portion is reduced in size, and the degreeof design freedom can be increased.

According to the sixth feature, said outlet pipe (60) includes asmaller-diameter portion (66) connected to an exhaust port of saidengine (E), said upstream oxygen sensor (U) being mounted on saidsmaller-diameter portion (66), and a bent portion (67) connected to saidsmaller-diameter portion (66) for bending a direction in which theexhaust gas is discharged rearwardly with respect to the vehicle bodyand connected to said larger-diameter portion (61), and said bentportion (67) has a recessed-projected portion (68) for dispersing theexhaust gas. Therefore, the bent portion of the outlet pipe makes itpossible to place the larger-diameter portion in a position closer tothe engine, allowing the catalyzer to exhibit its performancesufficiently. The recessed-projected portion disperses the exhaust gaswhich tends to gather in one side of the tubular passage due, to thebent portion, so that the exhaust gas can pass through the catalyzer inits entirety.

According to the eighth feature, said larger-diameter portion (61)includes a front portion as an outer shell (76) of an assembled hollowstructure and a rear portion (83) as an integral component, and saidcatalyzer (C) is disposed in said front portion. Therefore, though thenumber of parts used grows in manufacturing the larger-diameter portion,the ease with which to assemble the parts is increased by a process ofwelding three overlapping sheets. As the rear portion is in the form ofan integral component formed by a drawing process, a welding process, orthe like, the possibility that the exhaust gas that passes through theclearance of the assembled hollow structure in bypassing relation to thecatalyzer will flow downstream can be reduced.

According to the ninth feature, a catalyst diagnosing unit (72) iscomprised which is configured to diagnose and detect a deterioratedstate of said catalyzer (C) on the basis of sensor outputs from saidupstream oxygen sensor (U) and said downstream oxygen sensor (D),wherein when said catalyst diagnosing unit (72) diagnoses said catalyzer(C) as being deteriorated, an indicator (74) is activated to indicatethe deteriorated state to an occupant. Therefore, a deteriorated stateof the catalyzer is indicated to an occupant or the like by turning onor blinking the indicator, so that the vehicle is prevented fromtraveling on while the catalyzer is in the deteriorated state.

According to the tenth feature, said stand receiver (90) is fixed in aposition near a front end of a tapered portion (85) covering a jointpipe (84) for introducing the exhaust gas into said muffler (26).Therefore, the downstream oxygen sensor is protected using the standreceiver that is firmly fixed to receive the main stand when it isstored, and the downstream oxygen sensor is provided in a position nearthe front end of the tapered portion that surrounds the joint pipe foran enhanced appearance, making it easy for its sensor unit of thedownstream oxygen sensor to project into the joint pipe. The joint pipehas a small diameter and hence the possibility that the oxygenconcentration in the pipe will be biased is low compared with alarge-diameter portion on which the downstream oxygen sensor is mounted.Therefore, the detection accuracy with which the downstream oxygensensor is easy to maintain.

According to the eleventh feature, said stand device (22, 140) includesa gusset (133, 134, 135, 136, 150) increasing the rigidity of the standdevice (22, 140) and protecting said downstream oxygen sensor (D).Therefore, the functional component mounted on the stand device is ableto protect the downstream oxygen sensor more effectively.

According to the twelfth feature, said main stand (22) for abuttingagainst said stand receiver (90) has a portion including a rubber damper(130) mounted on a mount (22 d) fixed to a main pipe section (22 a) ofsaid main stand (22). Therefore, the rubber damper absorbs vibrationsproduced when the main stand is stored and during traveling, therebyreducing the possibility that those vibrations will affect thedownstream oxygen sensor.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a left-hand side elevational view of a motorcycleincorporating therein an exhaust device for motorcycle according to anembodiment of the present invention.

FIG. 2 is a right-hand side elevational view of the motorcycle.

FIG. 3 is a front elevational view of the motorcycle.

FIG. 4 is a rear elevational view of the motorcycle.

FIG. 5 is a left-hand side elevational view of an engine with theexhaust device mounted thereon.

FIG. 6 is a block diagram illustrating the relationship between theengine and oxygen sensors.

FIG. 7 is a flowchart of a catalyst deterioration detection controlsequence.

FIG. 8A and FIG. 8B are front elevational views of oxygen sensorsmounted on the exhaust device.

FIG. 9 is a plan view of a pipe section of the exhaust device.

FIG. 10 is an enlarged front elevational view of a larger-diameterportion.

FIG. 11 is a cross-sectional view taken along line XI-XI of FIG. 10.

FIG. 12A and FIG. 12B are cross-sectional views taken along line XII-XIIof FIG. 10.

FIG. 13 is a cross-sectional view taken along line XIII-XIII of FIG.12A.

FIG. 14 is a cross-sectional view taken along line XIV-XIV of FIG. 12A.

FIG. 15 is a cross-sectional view of a larger-diameter portion accordingto a modification of the second embodiment of the present invention.

FIG. 16 is a plan view of a muffler.

FIG. 17 is a side elevational view, partly in cross section, of themuffler.

FIG. 18 is a fragmentary enlarged view illustrating a bottom of themotorcycle.

FIG. 19 is a left-hand side elevational view illustrating the positionalrelationship between a main stand and a side stand, and the muffler.

FIG. 20 is a plan view illustrating the positional relationship betweenthe main stand and the side stand, and the muffler.

FIG. 21 is an enlarged cross-sectional view of a region where thedownstream oxygen sensor is mounted in position.

FIG. 22 is a view illustrating the positional relationship between astand receiver and a rubber damper.

FIG. 23 is a plan view of the main stand.

FIG. 24 is a plan view of a main stand according to a modification.

FIG. 25 is a plan view of a side stand according to a modification.

FIG. 26 is a left-hand side elevational view of the side stand.

FIG. 27 is a cross-sectional view taken along line XXVII-XXVII of FIG.25.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be described indetail below with reference to the drawings. FIG. 1 is a left-hand sideelevational view of a motorcycle 1 incorporating therein an exhaustdevice 20 according to an embodiment of the present invention. FIG. 2 isa right-hand side elevational view of the motorcycle 1. FIG. 3 is afront elevational view of the motorcycle 1. FIG. 4 is a rear elevationalview of the motorcycle 1.

The motorcycle 1 as a saddle-type vehicle includes a vehicle frame 2having a front end on which there is mounted a head pipe 12 with asteering stem 10 angularly movably supported thereon. A steering handle6 is installed on the upper end of the steering stem 10 by a top bridge,not illustrated. The top bridge, which is angularly movable in unisonwith the steering stem 10, and a bottom bridge, not illustrated, fixedto the steering stem 10 beneath the head pipe 12 jointly support a pairof left and right front fork legs 16. A front wheel WF provided with abrake disk 35 is rotatably supported on the lower ends of the front forklegs 16.

A parallel two-cylinder engine E whose cylinder head 18 has a lowerportion supported by a hanger frame 17 extending downwardly from behindthe head pipe 12 is disposed on a lower portion of the vehicle frame 2.A generator cover Ea and a drive sprocket cover Eb are mounted on a leftside of the engine E in a vehicle widthwise direction. A radiator 15 foran engine coolant is disposed in front of the hanger frame 17. Thevehicle frame 2 supports upper and lower portions of the engine E, andalso supports a swing arm 24 swingably through a pivot 21. A pair ofleft and right rider's footsteps 23 are provided below a pivot plate 21a on which the pivot 21 is supported, and foldable passenger's footsteps21 b are disposed on step brackets 21 c extending rearwardly andupwardly therefrom. Beneath the rider's footsteps 23, there areinstalled a main stand 22 for making the motorcycle 1 stand still on itsown with a rear wheel WR off the ground when the motorcycle 1 isstopped, and a side stand 140 for making the motorcycle 1 stand still onits own while the vehicle body of the motorcycle 1 is tilted to theleft. The main stand 22 and the side stand 140 are brought into a storedstate by swinging at approximately 90 degrees in a rearward direction ofthe vehicle body.

The exhaust device 20 for purifying and silencing an exhaust gas fromthe engine E and discharging the exhaust gas rearwardly is mounted on alower portion of the vehicle body of the motorcycle 1. The exhaustdevice 20 has a pipe section 19 connected to the exhaust port of thecylinders for guiding the exhaust gas rearwardly, and a muffler 26connected to the rear end of the pipe section 19. An exhaust pipe cover5 a that covers front and side portions of the pipe section 19 isdisposed forwardly and downwardly of the cylinder head 18. The swing arm24 supported swingably through the pivot 21 is suspended from thevehicle frame 2 by a rear cushion, not illustrated. Driving power fromthe engine E is transmitted by a drive chain 25 to the rear wheel WRwhich is rotatably supported on the rear end of the swing arm 24.

A storage box 4 that is accessible through a large-size openable andclosable lid 3 is provided in a position covered by side cowls 5 asouter covering members above the engine E. A headlight 13 is disposed infront of the side cowls 5, and a pair of direction indicating devices 11and a windscreen 9 are disposed above the headlight 13. Knuckle guards 8and rearview mirrors 7 are mounted on respective left and right steeringhandles 6. A pair of left and right fog lamps 14 are mounted inpositions on lower portion of the side cowls 5 outwardly of the frontfork legs 16 in vehicle widthwise directions. A front fender 36 forpreventing mud from splashing onto the vehicle body is installed abovethe front wheel WF.

A rear frame 29 that supports a fuel tank 28, etc. is mounted on a rearportion of the vehicle frame 2. The rear frame 29 has left and rightportions covered with a seat cowl 31 over which there are disposed arider's seat 27 and a passenger's seat 30. A tail light device 32 ismounted on the rear end of the seat cowl 31, and rear directionindicating devices 33 are supported on a rear fender 34 that extendsrearwardly and downwardly from the seat cowl 31.

As illustrated in FIGS. 3 and 4, a front brake lever 39 and a handleswitch 38 are mounted on the right steering handle 6, and a clutch lever41 and a handle switch 40 are mounted on the left steering handle 6. Ameter device 37 is disposed between the left and right steering handles6. A number plate holder 42 is mounted on the rear fender 34.

The main stand 22 mainly includes left and right main pipe sections 22 acurved in a substantially U shape and a reinforcing pipe 22 c couplingthe left and right main pipe sections 22 a. When stored, the main stand22 is kept in a storage position under the resiliency of a returnspring. To use the main stand 22, a foot step 22 b of the main stand 22is trod on to bring the distal ends of the main pipe sections 22 a intocontact with the ground, and while the foot step 22 b is being trod on,the rear portion of the vehicle body is lifted upwardly to cause themain stand 22 to swing into its usage position, making the vehicle bodystand still on its own with the rear wheel WR off the ground. A rubberdamper 130 has a mount 22 d provided on the main pipe section 22 a onthe right side of the vehicle body, of the main stand 22. When the mainstand 22 is stored, the rubber damper 130 is held in abutment against agiven portion of the muffler 26 for thereby absorbing shocks producedduring being stored and vibrations produced during traveling.

FIG. 5 is a left-hand side elevational view of the engine E with theexhaust device 20 mounted thereon. The engine E includes a paralleltwo-cylinder gasoline internal combustion engine with cylinders 43having a cylinder axis Os largely inclined forwardly of the vehicle bodywith respect to a vertical direction V. In FIG. 5, the generator coverEa and the drive sprocket cover Eb illustrated in FIG. 1 are omittedfrom illustration. The engine E is supported on the vehicle frame 2 byan upper hanger 50 a, a rear hanger 50 b, and a lower hanger 50 c of acrankcase 52, and suspended from the hanger frame 17 by a front hanger50 d formed on a cylinder block 43. The cylinder head 18 with an enginecoolant pump 44 mounted thereon is fixed to an upper portion of thecylinder block 43, and a cylinder head cover 45 is fixed to an upperportion of the cylinder head 18.

The engine E includes a crankshaft 46 having a left end to which analternating current generator (ACG) generator 48 is fixed for rotationin unison with the crankshaft 46. Rotary driving power from thecrankshaft 46 is transmitted to a transmission, not illustrated, made upof a plurality of gear pairs provided between a main shaft and acountershaft 49, delivered out from a drive sprocket 51 fixed to theleft end of the countershaft 49, and transmitted therefrom to the drivechain 25. An oil pan 54 is secured to the bottom of the crankcase 52,and an oil filter 53 is mounted on a front surface of the crankcase 52.

The exhaust device 20 has the pipe section 19 for guiding the exhaustgas rearwardly, and the muffler 26 connected to the rear end of the pipesection 19. The pipe section 19, which is made of a steel material suchas stainless steel or the like, includes an outlet pipe 60 connected tothe exhaust port of the cylinder head 18, a larger-diameter portion 61whose diameter is increased to house a catalyzer therein, and anintermediate pipe 62 coupled to the rear end of the larger-diameterportion 61 for guiding the exhaust gas to the muffler 26.

The pipe section 19 is positioned on the left side of the crankcase 52in the vehicle widthwise direction such that the larger-diameter portion61 lies clear of the oil filter 53 and the oil pan 54, and is curved tothe right side in a vehicle widthwise direction midway at theintermediate pipe 62 and connected to the muffler 26 disposed on theright side in the vehicle widthwise direction. A first oxygen sensor 100for detecting the oxygen concentration in the exhaust gas is mounted onthe outlet pipe 60 at a position immediately behind the junction withthe exhaust port. The larger-diameter portion 61 is positioned below thecylinder block 43 of the engine E and in front of the crankcase 52.

According to the present embodiment, the larger-diameter portion 61 hasan axis Oc substantially parallel to the cylinder axis Os of the engineE that are largely inclined forwardly of the vehicle body with respectto the vertical direction V. The outlet pipe 60 connected to thecylinder head 18 is bent from its junction substantially at a rightangle rearwardly and downwardly with respect to the vehicle body and isconnected to the larger-diameter portion 61.

FIG. 6 is a block diagram illustrating the relationship between theengine and oxygen sensors. FIG. 7 is a flowchart of a catalystdeterioration detection control sequence, and FIG. 8A and FIG. 8B arefront elevational views of oxygen sensors mounted on the exhaust device20. The exhaust device 20 has an upstream oxygen sensor U positionedupstream of a catalyzer C and a downstream oxygen sensor D positioneddownstream of the catalyzer C, for detecting deterioration of thecatalyzer C that is housed in the larger-diameter portion 61.

As illustrated in FIG. 8A and FIG. 8B, the oxygen sensors include leanair/fuel ratio (LAF) sensors capable of linearly detecting a change inthe oxygen concentration and oxygen sensors (O₂ sensors) capable ofdetecting only when the air-fuel ratio is a stoichiometric air-fuelratio because their output value is inverted as it changes across thestoichiometric air-fuel ratio. Of these sensors, the so-called oxygensensors that are relatively inexpensive compared with the expensive LAFsensors are often used on saddle-type vehicles such as motorcycles orthe like. Furthermore, the oxygen sensors include a heaterless oxygensensor 100 illustrated in FIG. 8A and a heater-mounted oxygen sensor 110illustrated in FIG. 8B.

The heaterless oxygen sensor 100 and the heater-mounted oxygen sensor110 are selectively used depending on whether the location where theycan be installed is more likely to be heated by engine heat.Specifically, the heaterless oxygen sensor 100 is used in a locationwhich is close to the engine E and that quickly reaches ahigh-temperature state wherein the zirconia element of the oxygen sensoris activated. On the other hand, the heater-mounted oxygen sensor 110that is more expensive is used in a position which is remote from theengine E and hence that requires the zirconia element to be positivelyheated by a heater.

The heaterless oxygen sensor 100 is mounted on an exhaust pipe 107 suchthat a sensor unit 104 thereof projects into the exhaust pipe 107, bythreading an externally threaded portion 103 into a mount boss 106 andfastening a nut 102 that is integral with a main body 101. A cable 105for outputting a sensor signal is connected to the end of the main body101. The heaterless oxygen sensor 100 has such features that it is of asimple structure, made up of a small number of parts, and of excellentdurability.

The heater-mounted oxygen sensor 110 is mounted on the exhaust pipe 107such that a sensor unit 114 thereof with a built-in ceramic heaterprojects into the exhaust pipe 107, by threading an externally threadedportion 113 into a mount boss 116 and fastening a nut 112 that isintegral with a main body 111, which is larger in diameter than theheaterless oxygen sensor 100. A cable 115 for outputting a sensor signalis connected to the end of the main body 111. The heater-mounted oxygensensor 110 has such features that the zirconia element thereof canquickly be activated even if it is far from a heat source, though it ismade up of a large number of parts and highly costly because of thebuilt-in heater, and slightly lower in water coverage resistance.

In the exhaust device 20 according to the present invention, asillustrated in the block diagram of FIG. 6, the heaterless oxygen sensor100 is mounted in a given position (the position illustrated in FIG. 5)upstream of the catalyzer C, and either one of the heaterless oxygensensor 100 and the heater-mounted oxygen sensor 110, depending on theposition where it is installed, is applied downstream of the catalyzerC.

Injectors 57 as fuel injection devices are provided on an intake pipe 56of the engine E, and an intake air rate sensor 55 is disposed upstreamthereof. A sensor signal from the intake air rate sensor 55 is input toan air rate detector 58. As described above, the upstream oxygen sensorU (heaterless oxygen sensor 100) is mounted on the outlet pipe 60connected to the exhaust port of the engine E. A sensor signal from theupstream oxygen sensor U is input to a first air/fuel ratio (A/F)controller 70.

An injector controller 59 controls the injectors 57 to burn at anappropriate air-fuel ratio on the basis of information representing athrottle action and an engine rotational speed and signals from the airrate detector 58 and the first A/F controller 70.

The downstream oxygen sensor D is provided downstream of the catalyzerC, and a sensor signal from the downstream oxygen sensor D is input to asecond A/F controller 71. The sensor signals from the upstream oxygensensor U and the downstream oxygen sensor D are input to a catalystdiagnosing unit 72. The catalyst diagnosing unit 72 diagnoses anddetects a deteriorated state of the catalyzer C on the basis of thesensor signals from the upstream oxygen sensor U and the downstreamoxygen sensor D, and transmits to an error processor 73. If thedeteriorated degree of the catalyzer C does not satisfy a predeterminedstandard, then the error processor 73 turns on or blinks an indicator74, indicating the error to the rider. The indicator 74 may be providedin the meter device 37 or the like of the motorcycle 1.

If the catalyst diagnosing unit 72 decides that the catalyzer C hasdeteriorated, then diagnostic information is input therefrom to thefirst A/F controller 70. In this case, for example, an output limitingcontrol process for reducing a fuel injection rate is carried out tosuppress the generation of substances to be purified by the catalyzer C.

The catalyst deterioration detection control sequence will be describedbelow with reference to the flowchart illustrated in FIG. 7. When theengine E starts to operate in step S1, the output of the upstream oxygensensor U is detected in step S2, and then the output of the downstreamoxygen sensor D is detected in step S3.

In step S4, it is determined whether the engine E has reached a stableoperating state. If the decision is affirmative in step S4, e.g., if thetemperature of the coolant of the engine E has reached a predeterminedtemperature, deciding that the warm-up operation is finished, then thesequence goes to step S5. If the decision is negative in step S4, thenthe sequence goes back to step S2.

In step S5, a catalyst deterioration diagnosing process is carried outbased on the two sensor outputs from the upstream oxygen sensor U andthe downstream oxygen sensor D. The catalyst deterioration diagnosingprocess focuses on the relationship between the sensor output of theupstream oxygen sensor U and the sensor output of the downstream oxygensensor D, and is performed by detecting a change caused by deteriorationof the catalyzer C. Specifically, a passive method such as a countingmethod for counting the number of times that the downstream oxygensensor D exhibits a predetermined change within a predetermined time, anamplitude ratio method for comparing the amplitudes of the sensor outputof the upstream oxygen sensor U and the sensor output of the downstreamoxygen sensor D with each other, an amplitude estimating method forestimating a behavior of the sensor output of the downstream oxygensensor D from a behavior of the sensor output of the upstream oxygensensor U and comparing the estimated behavior with an actually measuredvalue, or the like may be applied.

For example, according to a method that focuses on a reduction in theadsorption rate of oxygen due to deterioration of the catalyst, forfeedback-controlling the air-fuel ratio based on the output of thedownstream oxygen sensor, since the response time until the oxygenconcentration in the exhaust gas changes under the feedback controlvaries under the influence of deterioration, the deteriorated state ofthe catalyst can be determined by determining whether the period ofchange in the output of the downstream gas sensor falls under apredetermined catalyst deteriorating condition. According to thismethod, even when the internal combustion engine starts to operate at alow temperature, the feedback control of the air-fuel ratio can becarried out in an early stage, and the deteriorated state of thecatalyst can be determined. Aside from the above passive method, anactive method such as a Cmax method, a CMD method, or the like fordeliberately changing the oxygen concentration in the exhaust gas isalso applicable to the catalyst deterioration diagnosing process.

FIG. 9 is a plan view of the pipe section 19 of the exhaust device 20.FIG. 10 is an enlarged front elevational view of the larger-diameterportion 61, and FIG. 11 is a cross-sectional view taken along line XI-XIof FIG. 10. The outlet pipe 60 includes a smaller-diameter portion 66and a bent portion 67. The smaller-diameter portion 66 is of such ashape that it becomes gradually larger in diameter from an oblong inlethole 64 therein that matches the shape of the exhaust port and changesto a circular cross-sectional shape. The upstream oxygen sensor U ismounted on the right side of the smaller-diameter portion 66 in thevehicle widthwise direction and oriented outwardly from the center inthe vehicle widthwise direction, and disposed in a position less exposedto external view. The area from the smaller-diameter portion 66 up tothe larger-diameter portion 61 is covered with the exhaust pipe cover 5a illustrated in FIG. 1 that is disposed in front thereof, so that theupstream oxygen sensor U is made much less exposed to external view.

As illustrated in FIG. 11, a joint ring 69 is fixed to the distal end ofthe smaller-diameter portion 66 in abutment against a gasket by a weldbead B, and a flange 63 engages a downstream side thereof. The bentportion 67 is fixed to the rear end of the smaller-diameter portion 66by a weld bead B. A mount boss 106 for the upstream oxygen sensor U isfixed to the right side of the smaller-diameter portion 66 in thevehicle widthwise direction by a weld bead B.

The bent portion 67 is of such a shape that it is progressively greaterin diameter while bending the direction of the exhaust gas atapproximately 90 degrees from a forwardly downward direction to arearwardly downward direction in order to connect the smaller-diameterportion 66 and the larger-diameter portion 61 to each other. The bentportion 67 has a recessed portion 68 as a recessed-projected portionthat is recessed upwardly and rearwardly with respect to the vehiclebody. The recessed portion 68 provides a projected portion for repellingthe exhaust gas in many directions within the tubular passage, so thateven though the direction of the passageway of the exhaust gas is bentat approximately 90 degrees, the passageway of the exhaust gas is notbiased, but the exhaust gas can pass through the catalyzer C in itsentirety. The recessed portion 68 may be modified into any of variousshapes such as a projected shape, a wavy shape, etc. capable ofdispersing the exhaust gas.

The larger-diameter portion 61 that houses the catalyzer C therein isprovided rearwardly of the bent portion 67. The intermediate pipe 62 isconnected to the rear end of the larger-diameter portion 61. Accordingto the present embodiment, the portion which ranges from the bentportion 67 to the larger-diameter portion 61 is of a so-called“assembled hollow structure” made up of left and right parts joinedtogether. The assembled hollow structure has an upstanding mating region65 on upper and lower portions of the bent portion 67 and thelarger-diameter portion 61.

The upstream oxygen sensor U provided at outlet pipe 60 is positionedabove the central line Oc of the larger-diameter portion 61 with respectto the vehicle body as viewed in side elevation of the vehicle body, sothat upstream oxygen sensor U is protected from jumping pebbles and doesnot affect the bank angle of the motorcycle 1.

FIG. 12A and FIG. 12B are cross-sectional views taken along line XII-XIIof FIG. 10. As illustrated in FIG. 12A, the larger-diameter portion 61houses therein the catalyzer C in the form of a cylinder as a three-waycatalyst including a ceramics honeycomb. An annular packing 75 that canbe divided into two parts is enclosed between an outer shell 77 of thecatalyzer C and an outer shell 76 of the larger-diameter portion 61 forpreventing the exhaust gas from leaking rearwardly from between thoseouter shells.

As illustrated in FIG. 12B, the catalyzer C may be replaced with twocatalyzers C1 and C2 which exhibit different characteristics in upstreamand downstream regions. In this case, the downstream catalyzer C2 may beof such characteristics that it can be heated more easily than theupstream catalyzer C1, thereby shortening the time required for theoverall catalyzer assembly to reach an activating temperature.

FIG. 13 is a cross-sectional view taken along line XIII-XIII of FIG. 12.FIG. 14 is a cross-sectional view taken along line XIV-XIV of FIG. 12.As described above, the annular packing 75 that can be divided into twoparts is enclosed between the outer shell 77 of the catalyzer C and theouter shell 76 of the larger-diameter portion 61. The assembled hollowstructure of the outer shell 76 of the larger-diameter portion 61 hasthe mating region 65 of two parts fixed by the weld bead B from outside,and hence it is difficult to remove a clearance 79 formed on an innercircumferential surface. Even if the annular packing 75 includes acircular integral component, an exhaust gas that does not pass throughthe catalyzer C flows downstream through the clearance 79. A structureillustrated FIG. 15 may be employed to deal with such a difficulty.

FIG. 15 is a cross-sectional view illustrating the structure of alarger-diameter portion 61 according to a modification of the presentembodiment. According to the present modification, the larger-diameterportion 61 includes a front portion having an assembled hollow structureand a rear portion 83 in the form of an integral component formed by adrawing process or a winding process. Specifically, the rear end of theouter shell 76 of the larger-diameter portion 61 is shortened intopositional alignment with the rear end of the outer shell 77 of thecatalyzer C, and the rear portion 83 which is a separate component isfixed thereto by a process of so-called welding three overlappingsheets, thereby forming the larger-diameter portion 61. With thisconstruction, though the number of parts used grows, the ease with whichto assemble the parts is increased by the process of welding threeoverlapping sheets, and the possibility that the exhaust gas that passesthrough the clearance 79 of the assembled hollow structure will flowdownstream can be reduced.

FIG. 16 is a plan view of the muffler 26. FIG. 25 is a side elevationalview, partly in cross section, of the muffler 26. According to thepresent embodiment, the downstream oxygen sensor D includes aheater-mounted oxygen sensor 110 and is disposed in a position close tothe front end of the muffler 26. The muffler 26 which is connected tothe rear end of the intermediate pipe 62 includes a joint pipe 84, atapered portion 85 which is progressively larger in diameter from thejoint pipe 84, and a hollow cylindrical muffler portion 86. On thetapered portion 85, there are mounted mount stays 89 a and 89 b formounting a heat guard, not illustrated, thereon, and a stand receiver 90against which the rubber damper 130 of the main stand 22 will be held inabutment. A muffler stay 88 by which the muffler 26 is to be suspendedfrom the rear frame 29 is provided on an upper surface of the mufflerportion 86, and a muffler end 87 is provided on the rear end of themuffler portion 86.

According to the present embodiment, the upstream oxygen sensor U isprovided on the outlet pipe 60, and the downstream oxygen sensor D isprovided on the tapered portion 85 of the muffler 26. Since thisposition is spaced from the heat source and less liable to be heated,the heater-mounted oxygen sensor 110 is used. Furthermore, thedownstream oxygen sensor D is mounted on the inner side in the vehiclewidthwise direction such that the downstream oxygen sensor D isaccommodated in the stand receiver 90 which is formed of a plate-likemember bend into a substantially U-shaped cross section. The downstreamoxygen sensor D thus installed is less exposed to external view, andless subject to jumping pebbles and splashing water.

The stand receiver 90 is fixed in a position near the front end of thetapered portion 85 that covers the joint pipe 84 through which theexhaust gas is introduced into the muffler 26. Consequently, thedownstream oxygen sensor D is protected by the stand receiver 90 that isfirmly fixed in order to bear the main stand 22 when it is stored, andthe downstream oxygen sensor D that is provided in a position near thefront end of the tapered portion 85 that covers the periphery of thejoint pipe 84 for an enhanced appearance makes it easy for its sensorunit of the downstream oxygen sensor D to project into the joint pipe84.

The portion on which the stand receiver 90 is mounted includes a slenderpipe, and hence the possibility that the oxygen concentration in thepipe will be biased is low compared with a large-diameter portion suchas the larger-diameter portion 61 on which the downstream oxygen sensorD is mounted. Therefore, the detection accuracy of the downstream oxygensensor D is easy to maintain.

As illustrated in FIG. 17, the front end of the joint pipe 84 has aflange 91 by which the joint pipe 84 is connected to the intermediatepipe 62. The joint pipe 84 has a rear end extending through the taperedportion 85 and inserted through a first partition 92 into a firstexpansion chamber 93. A second partition 94 is provided at the rear endof the first expansion chamber 93, dividing the first expansion chamber93 from a second expansion chamber 96 through which a second pipe 95extends. A third partition 97 supports the second pipe 95 and a thirdpipe 99, and is provided as an upstanding partition between the secondexpansion chamber 96 and a third expansion chamber 98. The exhaust gasthat is introduced from the joint pipe 84 is silenced as it passesthrough the expansion chambers, and then discharged from the rear end ofthe muffler 26. The muffler 26 thus silences the exhaust gas stepwise,and makes the exhaust gas less liable to rise in temperature as it flowsdownstream. According to the present embodiment, inasmuch as thedownstream oxygen sensor D is disposed closely to the front end of thejoint pipe 84 which is most upstream in the muffler 26, the burden onthe heater is held to a minimum and the downstream oxygen sensor D canbe quickly activated.

FIG. 18 is a fragmentary enlarged view illustrating the bottom of themotorcycle 1. FIG. 19 is a left-hand side elevational view illustratingthe positional relationship between the main stand 22 and the side stand140, and the muffler 26, and FIG. 20 is a plan view illustrating thepositional relationship between the main stand 22 and the side stand140, and the muffler 26. According to the present embodiment, the mainstand 22 or the side stand 140 makes up a stand device which protects alower portion or a front and lower portion of the downstream oxygensensor D.

The intermediate pipe 62 extends from the left side of the vehicle bodyacross the central line O of the vehicle body to the right side of thevehicle body, staying clear of the oil pan 54 of the engine E and themain stand 22, and is connected to the muffler 26. A clamp 120 forpreventing an exhaust leakage is wound around the flange 91 thatconnects the intermediate pipe 62 and the joint pipe 84 to each other.The swing arm 24 is suspended from the vehicle frame 2 by a rear cushion121 through a link mechanism 122. The main stand 22 has a swing shaft125 disposed in front of the link mechanism 122. A brake pedal 123 isdisposed in front of the right footstep 23, whereas a shift pedal 124 isdisposed in front of the left footstep 23.

The main stand 22 is swingably supported on the vehicle frame 2 by theswing shaft 125. The main pipe sections 22 a of the main stand 22 arecurved into a substantially U shape, and have on their ends groundingsoles 22 e which touch the ground when the main stand 22 is in use. Agusset 22 f which is welded to the main pipe section 22 a for increasedrigidity is provided on the proximal portion of the foot step 22 b thatis provided on the left side of the vehicle body in the vehiclewidthwise direction.

The mount 22 d of the rubber damper 130 is provided in the vicinity ofthe grounding sole 22 e on the right side in the vehicle widthwisedirection. The rubber damper 130 is held in abutment against the standreceiver 90 when the main stand 22 is stored. When the main stand 22 isstored, the stand receiver 90 has a lower surface covered with the mount22 d of the rubber damper 130 from below the vehicle body. The sidestand 140 is swingably supported on the left side of the vehicle frame 2in the vehicle widthwise direction by a swing shaft 147 that ispositioned forwardly of the swing shaft 125 of the main stand 22.

FIG. 21 is an enlarged cross-sectional view of a region where thedownstream oxygen sensor D is mounted in position. FIG. 22 is a viewillustrating the positional relationship between the stand receiver 90and the rubber damper 130. The downstream oxygen sensor D is mounted inposition by a screw threaded into the mount boss 116 that is fixed inposition by extending through the tapered portion 85 and the joint pipe84. As illustrated in FIG. 22, the stand receiver 90 has wall surfaces90 c on front and rear ends of a bottom surface 90 a against which therubber damper 130 is brought into abutment, and an extension 90 bthrough which the stand receiver 90 is welded to the tapered portion 85.The downstream oxygen sensor D is disposed so as to be housed in thestand receiver 90, making it possible to protect the front, rear, andlower sides of the downstream oxygen sensor D.

FIG. 23 is a plan view of the main stand 22. The swing shaft 125 issupported by a swing shaft pipe 22 h that is joined to the main pipesections 22 a of the main stand 22 by a front support 22 i and a pair ofleft and right rear supports 22 g. The left and right main pipe sections22 a are connected to each other by the reinforcing pipe 22 c. Asdescribed above, the downstream oxygen sensor D is protected by beingmounted in place so as to be surrounded by the stand receiver 90 and bythe stand receiver 90 covered with the mount 22 d of the rubber damper130. Furthermore, the downstream oxygen sensor D can be protectedsecurely by adding improvements to the main stand 22.

Specifically, the main stand 22 is provided with gussets 133, 134, and135 illustrated hatched for receiving jumping pebbles (chipping) orsplashing water forwardly of the downstream oxygen sensor D with respectto the vehicle body to prevent them from affecting the downstream oxygensensor D. The gussets 133, 134, and 135 have a function to not onlyserve as part of the main stand 22 for protecting the downstream oxygensensor D, but also increase the rigidity with which the main pipesections 22 a and the rear supports 22 g of the main stand 22 are joinedto each other and the rigidity with which the main pipe sections 22 aand the reinforcing pipe 22 c of the main stand 22 are joined to eachother.

FIG. 24 is a plan view of a main stand 22 according to a modification.If the muffler 26 is disposed on the left side of the vehicle body, noton the right side of the vehicle body, so that the downstream oxygensensor D mounted on the muffler 26 is also positioned on the left sideof the vehicle body, then gussets may be provided on the left side ofthe vehicle body for protecting the downstream oxygen sensor D. Gussets134 a and 135 a are shaped in symmetrical relation to the gussets 134and 135 illustrated in FIG. 23. According to the present modification,moreover, a triangular gusset 136 is provided between the reinforcingpipe 22 c and one of the grounding sole 22 e to cover the underside ofthe downstream oxygen sensor D. Since the gussets mounted on the mainstand 22 are components of the main stand 22 for increasing the rigidityof the respective parts, it is not necessary to provide separatededicated components for protecting the downstream oxygen sensor D, andhence the production cost is prevented from increasing and the space iseffectively utilized.

FIG. 25 is a plan view of a side stand 140 according to a modification.FIG. 26 is a left-hand side elevational view of the side stand 140, andFIG. 27 is a cross-sectional view taken along line XXVII-XXVII of FIG.25. The downstream oxygen sensor D may be protected by the side stand140. If the side stand 140 is provided on the left side of the vehiclebody as with the present embodiment, it is suitable for protecting thedownstream oxygen sensor D that is disposed close to the left side ofthe vehicle body, and if the side stand 140 is provided on the rightside of the vehicle body, it is suitable for protecting the downstreamoxygen sensor D that is disposed close to the right side of the vehiclebody. FIGS. 25 through 27 illustrate a structure wherein theintermediate pipe 62 extends on the left side of the vehicle body andthe downstream oxygen sensor D mounted on the intermediate pipe 62 isprotected by the side stand 140 on the left side of the vehicle body.

The side stand 140 includes a main pipe section 141 connected to the endof a support 142 through which a swing shaft 147 extends, and agrounding sole 145 mounted on the end of the main pipe section 141. Arod-shaped actuator 144 which the rider places its foot on to pull outthe side stand 140 is provided on the left side of the main pipe section141 in the vehicle widthwise direction, and a return spring hook 146 isprovided on a side of the main pipe section 141 which is close to thecenter of the vehicle body.

When the side stand 140 is stored, the support 142 is held in abutmentagainst a stopper on the vehicle frame, and hence the stopper serves asa stand receiver. Alternatively, a rubber damper may be mounted on themain pipe section 141 and a stand receiver may be formed on the vehiclebody for protecting the downstream oxygen sensor D.

As illustrated in FIG. 19, when stored, the side stand 140 is positionedupwardly of the main stand 22 that is stored as viewed in side elevationof the vehicle body. Therefore, the side stand 140 can possibly protectthe downstream oxygen sensor D in a position closer thereto. Accordingto the present embodiment, a substantially triangular gusset 150 isprovided on an inner side of the main pipe section 141 in the vehiclewidthwise direction for receiving jumping pebbles and splashing waterfrom below the vehicle body.

As illustrated in FIG. 27, the gusset 150 may be mounted on a lowersurface of the main pipe section 141 of the side stand 140. In thiscase, the main pipe section 141 and the intermediate pipe 62 can bedisposed more closely to each other than if the gusset 150 is mounted onan upper surface of the main pipe section 141. With this arrangement,even if the downstream oxygen sensor D is not only mounted horizontallyon the intermediate pipe 62, but also mounted obliquely thereon at apredetermined angle upwardly from the horizontal direction with respectto the vehicle body, the downstream oxygen sensor D can be protectedfrom splashing water and mud by the gusset 150 that is present in arelatively close position. The shape and installed position of thegusset 150 and the positional relationship between the side stand 140and the intermediate pipe 62, etc. are not limited to the details andlayout described above, but may be modified in various ways. Forexample, the downstream sensor mounted on the muffler may be protectedby the side stand. The downstream oxygen sensor D may be protected notonly by the stand device, but by a plate member or the like extendingfrom the step bracket 21 c or the passenger's footstep 21 b.

The type of the engine to which the exhaust device is applied, the shapeand structure of the exhaust device, the structures of the oxygensensors, the positions where the oxygen sensors are disposed, and thespecific method of detecting deterioration of the catalyst with the twooxygen sensors, etc. are not limited to those illustrated in the aboveembodiments, but may be modified in various ways. For example, one orboth of the oxygen sensors may be an LAF sensor. The exhaust deviceaccording to the present invention is applicable to various vehiclesincluding saddle-type three-wheeled or four-wheeled vehicles or thelike, not only motorcycles.

REFERENCE SIGNS LIST

-   -   1 . . . motorcycle (vehicle),    -   19 . . . pipe section,    -   20 . . . exhaust device,    -   22 . . . main stand,    -   22 a . . . main pipe section,    -   22 d . . . mount,    -   26 . . . muffler,    -   43 . . . cylinder block,    -   52 . . . crankcase,    -   60 . . . outlet pipe,    -   61 . . . larger-diameter portion,    -   62 . . . intermediate pipe,    -   65 . . . mating region,    -   66 . . . smaller-diameter portion,    -   67 . . . bent portion,    -   68 . . . recessed portion (recessed-projected portion),    -   76 . . . outer shell,    -   83 . . . rear portion,    -   85 . . . tapered portion,    -   90 . . . stand receiver,    -   90 a . . . bottom surface,    -   90 c . . . upstanding wall surface,    -   100 . . . heaterless oxygen sensor,    -   110 . . . heater-mounted oxygen sensor,    -   130 . . . rubber damper,    -   133, 134, 135, 136, 150 . . . gusset,    -   140 . . . side stand,    -   C . . . catalyzer,    -   U . . . upstream oxygen sensor,    -   D . . . downstream oxygen sensor

The invention claimed is:
 1. An exhaust device for a motorcycle having apipe section extending below an engine for guiding an exhaust gastherefrom rearwardly with respect to a vehicle body and housing acatalyzer therein, and a muffler connected to a rear end of said pipesection, comprising: an upstream oxygen sensor positioned upstream ofsaid catalyzer and a downstream oxygen sensor positioned downstream ofsaid catalyzer, wherein said downstream oxygen sensor is disposed ineither a position overlapping a stand device for supporting a vehicle asviewed from below the vehicle body when said stand device is stored, ora position overlapping a stand receiver for abutting against said standdevice as viewed from below the vehicle body when said stand device isstored, said pipe section includes a larger-diameter portion having anincreased diameter for housing said catalyzer therein, saidlarger-diameter portion is positioned below a cylinder block of saidengine and in front of a crankcase of said engine, said engine has acylinder whose cylinder axis is inclined forwardly of the vehicle bodywith respect to a vertical direction, said larger-diameter portion isdisposed such that said larger-diameter portion has an axissubstantially parallel to said cylinder axis, said upstream oxygensensor and said downstream oxygen sensor are mounted outwardly from theinsides of said pipe section and said muffler in the vehicle widthwisedirection, said downstream oxygen sensor is positioned to overlap withboth of a part of said stand device and said stand receiver as viewedfrom below the vehicle body, and the front, rear and lower sides of saiddownstream oxygen sensor are covered with a plate-like member.
 2. Theexhaust device for a motorcycle according to claim 1, wherein saidupstream oxygen sensor includes a heaterless oxygen sensor free of aheater function and said downstream oxygen sensor includes aheater-mounted oxygen sensor with a heater function.
 3. The exhaustdevice for a motorcycle according to claim 1, wherein said stand deviceincludes a main stand, and said stand receiver is provided on saidmuffler.
 4. The exhaust device for a motorcycle according to claim 2,wherein said stand device includes a main stand, and said stand receiveris provided on said muffler.
 5. The exhaust device for a motorcycleaccording to claim 1, wherein said stand receiver includes theplate-like member, said plate-like member having upstanding wallsurfaces disposed on front and rear ends of a bottom surface and havinga substantially U-shaped cross section.
 6. The exhaust device for amotorcycle according to claim 2, wherein said stand receiver includesthe plate-like member, said plate-like member having upstanding wallsurfaces disposed on front and rear ends of a bottom surface and havinga substantially U-shaped cross section.
 7. The exhaust device for amotorcycle according to claim 2, wherein said upstream oxygen sensor ismounted on an outlet pipe positioned upstream of said larger-diameterportion.
 8. The exhaust device for a motorcycle according to claim 7,wherein said outlet pipe includes a smaller-diameter portion connectedto an exhaust port of said engine, said upstream oxygen sensor beingmounted on said smaller-diameter portion, and a bent portion connectedto said smaller-diameter portion for bending a direction in which theexhaust gas is discharged rearwardly with respect to the vehicle bodyand connected to said larger-diameter portion, and said bent portion hasa recessed-projected portion for dispersing the exhaust gas.
 9. Theexhaust device for a motorcycle according to claim 1, wherein saidlarger-diameter portion includes a front portion as an outer shell of anassembled hollow structure and a rear portion as an integral component,and said catalyzer is disposed in said front portion.
 10. The exhaustdevice for a motorcycle according to claim 1, characterized incomprising: a catalyst diagnosing unit configured to diagnose and detecta deteriorated state of said catalyzer on the basis of sensor outputsfrom said upstream oxygen sensor and said downstream oxygen sensor,wherein when said catalyst diagnosing unit diagnoses said catalyzer asbeing deteriorated, an indicator is activated to indicate thedeteriorated state to an occupant.
 11. The exhaust device for amotorcycle according to claim 2, characterized in comprising: a catalystdiagnosing unit configured to diagnose and detect a deteriorated stateof said catalyzer on the basis of sensor outputs from said upstreamoxygen sensor and said downstream oxygen sensor, wherein when saidcatalyst diagnosing unit diagnoses said catalyzer as being deteriorated,an indicator is activated to indicate the deteriorated state to anoccupant.
 12. The exhaust device for a motorcycle according to claim 1,wherein said stand receiver is fixed in a position near a front end of atapered portion covering a joint pipe for introducing the exhaust gasinto said muffler.
 13. The exhaust device for a motorcycle according toclaim 2, wherein said stand receiver is fixed in a position near a frontend of a tapered portion covering a joint pipe for introducing theexhaust gas into said muffler.
 14. The exhaust device for a motorcycleaccording to claim 1, wherein said stand device includes a gussetincreasing the rigidity of the stand device and protecting saiddownstream oxygen sensor.
 15. The exhaust device for a motorcycleaccording to claim 2, wherein said stand device includes a gussetincreasing the rigidity of the stand device and protecting saiddownstream oxygen sensor.
 16. The exhaust device for a motorcycleaccording to claim 1, wherein said main stand for abutting against saidstand receiver has a portion including a rubber damper mounted on amount fixed to a main pipe section of said main stand.
 17. The exhaustdevice for a motorcycle according to claim 2, wherein said main standfor abutting against said stand receiver has a portion including arubber damper mounted on a mount fixed to a main pipe section of saidmain stand.
 18. The exhaust device for a motorcycle according to claim1, wherein said stand receiver includes said U-shaped plate-like memberwhich projects into inside of the vehicle body from said muffler so asto cover the front, rear and lower sides of said downstream oxygensensor.
 19. The exhaust device for a motorcycle according to claim 18,wherein said upstanding wall surfaces disposed on front and rear ends ofsaid bottom surface is inclined rearwardly of the vehicle body as viewedfrom a side of the vehicle body.